Observation of Floquet-Bloch States on the Surface of a Topological Insulator

TLDR

The surface electrons of a topological insulator are protected by time‑reversal symmetry, and circularly polarized light can break this symmetry, potentially inducing an exotic surface quantum Hall state. Time‑ and angle‑resolved photoemission spectroscopy reveals that an intense ultrashort mid‑infrared pulse hybridizes with surface Dirac fermions to create Floquet‑Bloch bands with polarization‑dependent gaps, including a Dirac‑point gap from circularly polarized photons, demonstrating broken time‑reversal symmetry and enabling optical control of topological quantum states.

Abstract

The unique electronic properties of the surface electrons in a topological insulator are protected by time-reversal symmetry. Circularly polarized light naturally breaks time-reversal symmetry, which may lead to an exotic surface quantum Hall state. Using time- and angle-resolved photoemission spectroscopy, we show that an intense ultrashort mid-infrared pulse with energy below the bulk band gap hybridizes with the surface Dirac fermions of a topological insulator to form Floquet-Bloch bands. These photon dressed surface bands exhibit polarization-dependent band gaps at avoided crossings. Circularly polarized photons induce an additional gap at the Dirac point, which is a signature of broken time-reversal symmetry on the surface. These observations establish the Floquet-Bloch bands in solids and pave the way for optical manipulation of topological quantum states of matter.